The present invention relates to a silicone rubber composition, and more particularly the present invention relates to a silicone elastomeric composition which has a tear strength of over 100 pounds per inch.
There has been much innovative work done on heat curable, silicone rubber compositions. Basically, such compositions comprise a diorganopolysiloxane gum having a viscosity of at least 1,000,000 centipoise at 25.degree. C., silica filler, process aids, and a peroxide free radical initiator. See, for instance, DeZuba et al., U.S. Pat. No. 3,730,932. In such compositions, it became necessary to vary the diorganopolysiloxane polymer. Particularly, it was desirable that the diorganopolysiloxane polymer contain vinyl radicals so that it could polymerize more readily into a elastomeric mass with desirable properties. Accordingly, one of the developments in the area was to vary the vinyl unsaturation in the polymer blend, and particularly have a polymer blend made from various kinds of vinyl-containing polymers; see, for instance, U.S. Pat. No. 3,660,345.
Another development was to make a high viscosity, heat-curable silicone rubber composition comprised of vinyl-containing polysiloxanes, a hydride-containing polysiloxane cross-linking agent, and a platinum catalyst with filler and other ingredients. The novel ingredient in such compositions was the use of a hydroperoxide inhibitor which will allow the composition to have a long work life in one instance. In another instance, it allows the compositions to be packaged in a single unit and upon heating the composition at elevated temperatures, i.e., temperatures above 100.degree. C., the composition cured to a silicone elastomeric mass. Such a development is disclosed in Bobear, U.S. Pat. No. 4,061,609. All the patents set forth in the instant case are incorporated by reference.
One variation on the above compositions is, for instance, to be found in Wada et al., U.S. Pat. No. 3,671,480, which discloses a vinyl blend as the base polymers with a silicone hydride and a platinum catalyst. In this patent, the vinyl blend comprises one polymer containing from 0.02 to 0.2 mole percent of vinyl-containing siloxy units which appears to have a high molecular weight, and another vinyl-containing polymer containing at least about 2 mole percent of vinyl-containing siloxy units of somewhat lower molecular weight. From the recitation in the patent, it appears the molecular weight of the second polymer, at the minimum, could be below 100,000 centipoise at 25.degree. C. The purpose of this patent was to produce a high-tear composition. It should be noted that the tear obtained by the samples set forth in the examples varied around 50 pi.
Another patent of interest is Polmanteer et al., U.S. Pat. No. 3,697,473, which describes a heat-curable composition comprising a vinyl siloxane, a hydride polysiloxane composed of various polymeric hydrides, and a platinum catalyst. The invention in this case was in the hydride blend. In this case the tear of the sample was as high in some cases as 200 p.i. However, the method of obtaining such high tear strength appears to have been the use of the novel hydride mixture of Polmanteer et al.
Further developments are disclosed in Wada et al., U.S. Pat. No. 3,652,475, which discloses a heat-curable silicone composition comprising a vinyl-containing polysiloxane of high viscosity, and containing at most 0.3 mole percent of vinyl-containing siloxy units, which was blended with a high viscosity vinyl-containing siloxane of at least the same viscosity as the other polymer, and containing from 5 to 20 mole percent of vinyl-containing siloxy units. Thirdly, there was present another polydiorganosiloxane of a smaller degree of polymerization containing from 5 to 90 mole percent of vinyl-containing siloxy units. Specifically, the vinyl-containing polysiloxane could be of very low molecular weight such that it could easily have a viscosity in the range of 1000 to 100,000 centipoise at 25.degree. C. within most of the degree of polymerization specified in the patent. This composition included silica filler and was cured by an organic peroxide. The tear strength of such compositions as set forth in the Examples was less than 50 p.i., especially after the post cure.
Another pertinent patent in this area is U.S. Pat. No. 3,696,068. This patent discloses a heat-curable silicone stock comprising a vinyl-containing diorganopolysiloxane gum, a linear vinyl-containing fluid having a viscosity from 10 to 150,000 centipoise at 25.degree. C., and further including a vinyl-containing resin. This composition as disclosed was cured with peroxides. The tear strength shown in the examples, and particularly, Table 1, Table 2, and Table 3, exceeded 100 p.i. in some cases, but in most cases, did not exceed 100 p.i. Tables 2 and 3 disclose samples which had tears in excess of 150 p.i. Again, this composition contained a low viscosity vinyl-containing fluid. However, it does not appear that the inventor in U.S. Pat. No. 3,669,068 carried out any measure of the tear of the samples after post-cure. The reason this is mentioned is that with a low vinyl-containing fluid in the composition, it has been found that after post-cure, the tear strength dramatically drops.
A patent application which is of interest is that of Bobear, Ser. No. 203,849, filed on Nov. 4, 1980, now abandoned, which discloses as the basic composition a vinyl-containing polysiloxane which can be a blend of polysiloxanes, a hydride polysiloxane, a platinum catalyst, and an inhibitor compound having at least one hydroperoxide radical. It is also disclosed that there can be an organic peroxide accelerator compound in the composition so as to accelerate the cure of the composition at elevated temperatures. It should be noted that the vinyl-containing polysiloxane disclosed in the specification can be as disclosed a blend of different vinyl-containing polysiloxanes. There is not disclosed what those types of vinyl-containing polysiloxanes can be or what the advantages of different types of vinyl-containing polysiloxanes can be in the composition.
Another patent of interest is that of Bobear, Ser. No. 106,795, filed on Dec. 26, 1979, now U.S. Pat. No. 4,288,360, which disclosed a flame retardant, heat-curable composition comprising an organopolysiloxane gum, a filler, a platinum compound and various amounts of a hydrated aluminum, titanium dioxide and magnesium oxide.
A more pertinent patent is that of Bobear et al., Ser. No. 156,137, filed on June 3, 1980, now abandoned, which discloses an SiH-olefin platinum catalyzed, high viscosity composition containing therein a silanol material which helps regulate the cure of the composition. The composition of the foregoing application Ser. No. 156,137 and Bobear U.S. Pat. No. 4,061,609, were effective in producing compositions whose cure could be regulated or inhibited and were desirable in that they had sufficient work life and could be inhibited so as to be stored in a one-component package. In addition, by varying the vinyl blend in accordance with U.S. Pat. No. 3,660,345, desirable tear strength properties would be obtained.
These compositions were very desirable, in that they could be hot air vulcanized, that is, they could be cured by hot air making them very desirable for medicinal tubing and other uses. However, the tear-strength properties of such compositions were limited, especially when the composition was post-cured as was desirable in most cases.
There was some experimentation with the variation of the vinyl blend to increase the tear-strength properties of the composition in accordance with Bobear, U.S. Pat. No. 3,660,345 with some success. However, it has now been unexpectedly found that by having a particular type of vinyl blend of two high viscosity polymers, a filler, a peroxide curing agent, and a hydride polysiloxane present, that the composition can cure to a silicone elastomer with exceedingly high tear strengths (in most all cases above 100 p.i. and in some cases exceeding 200 p.i.). This was achieved without having a complicated hydride blend which is difficult to make due to the natural instability of the hydride polysiloxanes or by utilizing a low viscosity vinyl fluid which it was later found would result in a composition whose post-cure tear strength would degrade dramatically.
Accordingly, it is one object of the present invention to provide for a high-tear-strength silicone rubber composition comprising a vinyl-containing polysiloxane gum blend.
It is another object of the present invention to provide for a polysiloxane composition that cures to a silicone elastomer having a tear strength after post-cure of above 100 p.i. which is formed from a high viscosity vinyl-containing polysiloxane blend, a hydride polysiloxane, and a peroxide curing agent.
It is an additional object of the present invention to provide a process for forming a heat curing silicone elastomeric composition having a tear strength of above 100 p.i. which is formed from a vinyl-containing polysiloxane blend, a hydride polysiloxane, and a peroxide curing agent.
It is yet still a further object of the present invention to provide medical tubing and spark plug boots which have a tear strength of above 100 p.i. which are formed from a high viscosity polysiloxane blend, a hydride polysiloxane cross-linking agent, and a peroxide curing agent.
These and other objects of the present invention are accomplished by means of the disclosures set forth herein below.